By means of various continuous tube drawing processes, the most popular of which are the Danner process, the Vello process, and the A drawing process, glass tubing has hitherto been produced with only a circular cross-section.
In the Danner process, molten glass which has been finished in a tank flows through a feeder channel to arrive through a nozzle on a mandrel usually formed by a ceramic cylinder, the mandrel being in a muffle furnace and comprising an axial passage. The mandrel is slightly inclined and rotates during the drawing process. The glass exiting from the nozzle is continuously wound onto the ceramic cylinder. Because of the slight inclination of the mandrel, the glass layer flows slowly towards a downstream mandrel head and is drawn off therefrom as a continuous tube by a drawing machine disposed at a certain distance. Air can be blown through the axial passage. By varying the air flow and the drawing velocity, as well as by changing the temperature in the muffle furnace, tubes of different diameters and wall thicknesses can be manufactured.
In the Vello process, the glass which has been finished in a tank flows through a feeder channel into a bowl, with the temperature being decrease so far that tubes or rods can be drawn from the bowl. The bowl has below an outflow ring with a circular opening. A downwardly enlarging needle is situated below or within that opening. The needle has a shaft which projects upwardly through the outflow ring and glass mass and is secured above the glass level in a needle holder so as to be displaceable, for purposes of precise centering, in all three spatial directions. Air can be blown through an axial passage of the needle. The glass flows with a tubular cross-section through the outflow ring and around the needle firstly downwards and then is deflected into the horizontal. At a certain distance, a drawing machine is disposed which draws the formed tubing continuously off. In accordance with changes of the temperature of the glass, the blowing air pressure, the mass flow and the drawing velocity, different dimensions of the tubing can be set.
The down drawing process corresponds to the Vello process, however, with the difference that the formed tubular glass is not deflected into the horizontal but, rather, is drawn vertically downwards. The drawing temperatures are somewhat lower than with the Vello process. With this process, larger tubing can be manufactured than with the Vello process.
Hirsch U.S. Pat. No. 1,574,842 describes such a process which, however, is not workable in the described manner. Therefore, the process was never used.
A manufacture of tubing the wall of which comprises a profile of some kind has not yet been carried out according to the aforementioned processes because prejudices existed among skilled people with respect to the practicability, as can be seen for instance from the printed publications Sprechsaal 114, Vol. 5/81, pp. 340-343; Glastechn. Ber. 54 (1981), No. 5, pp. 131-135; Journal of Non-crystalline Solids 7 (1972), pp. 203-220; and Journal of Non-Crystalline Solids 6 (1971), pp. 377-388, by E. Roeder and E. Roeder et al., respectively.
In these printed publications, it is set forth in connection with an extrusion process which is described there, that for producing sharp-edged profiles, a lowering of the working temperature and thus a high viscosity is necessary. As stated there, a rounding of edges would result from the surface tension in the conventional continuous tube drawing processes due to their substantially higher working temperatures and thus lower viscosities of the glass.
As further prejudices, it is believed that the glass would not flow through narrow gaps due to its high viscosity, that plane surfaces would not be obtainable, and that the profile would not reproducible. Moreover, it was believed that narrow tolerances could be achieved only by redrawing, calibrating or shrinking.
In the manufacture of profiled glass tubing, a success was achieved only with the extrusion process as known from the German disclosure letter 35 16 920. By application of a combined extrusion and drawing-through process, fiber-reinforced rods and solid profiles of inorganic glasses were manufactured, or of glasses which can be transferred into a glass ceramic with a core zone thereof being reinforced by continuous fibers.
As can be derived from Sprechsaal 114, Vol. 5/81, pp. 340-343; Glastechn. Ber. 54 (1981), No. 5, pp. 131-135, Journal of Non-Crystalline Solids 7 (1972), pp. 202-220; and Journal of Non-Crystalline Solids (1971), pp. 377-388, by E. Roeder and E. Roeder et al., respectively, also tubing of any desired inner and outer profiles can be produced with this extrusion process. However, that process suffers from the disadvantages that only shaped parts of restricted dimensions can be manufactured, and that the process is too expensive, from a cost standpoint, due to the necessary high pressures, to achieve importance beyond an application in special fields, for mass production.